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<a name="Avoiding-MPI-Deadlocks"></a>
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Next: <a href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" accesskey="n" rel="next">FFTW MPI Performance Tips</a>, Previous: <a href="FFTW-MPI-Wisdom.html#FFTW-MPI-Wisdom" accesskey="p" rel="prev">FFTW MPI Wisdom</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Avoiding-MPI-Deadlocks-1"></a>
<h3 class="section">6.9 Avoiding MPI Deadlocks</h3>
<a name="index-deadlock"></a>

<p>An MPI program can <em>deadlock</em> if one process is waiting for a
message from another process that never gets sent.  To avoid deadlocks
when using FFTW&rsquo;s MPI routines, it is important to know which
functions are <em>collective</em>: that is, which functions must
<em>always</em> be called in the <em>same order</em> from <em>every</em>
process in a given communicator.  (For example, <code>MPI_Barrier</code> is
the canonical example of a collective function in the MPI standard.)
<a name="index-collective-function-2"></a>
<a name="index-MPI_005fBarrier"></a>
</p>

<p>The functions in FFTW that are <em>always</em> collective are: every
function beginning with &lsquo;<samp>fftw_mpi_plan</samp>&rsquo;, as well as
<code>fftw_mpi_broadcast_wisdom</code> and <code>fftw_mpi_gather_wisdom</code>.
Also, the following functions from the ordinary FFTW interface are
collective when they are applied to a plan created by an
&lsquo;<samp>fftw_mpi_plan</samp>&rsquo; function: <code>fftw_execute</code>,
<code>fftw_destroy_plan</code>, and <code>fftw_flops</code>.
<a name="index-fftw_005fexecute-4"></a>
<a name="index-fftw_005fdestroy_005fplan-2"></a>
<a name="index-fftw_005fflops-1"></a>
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